1. Field of the Invention
The present invention relates to a control system for an automatic transmission for a vehicle, and more particularly, to a control system for an automatic transmission for a vehicle, wherein the shifting of the automatic transmission is prohibited when a slipping state of the vehicle is determined.
2. Description of the Prior Art
When a vehicle provided with a vehicle speed detecting means for detecting a vehicle speed on the basis of a rotational speed of a rotatable member of an automatic transmission, is started on ice having a small friction coefficient, the rotational speed of the rotatable member is increased if drive wheels slip. For this reason, the vehicle speed detecting means detects a higher vehicle speed, but the actual vehicle speed remains lower, because a gripping between the drive wheels and the road surface has been lost. In this case, because the vehicle speed detecting means detects the higher vehicle speed, there is a possibility that the automatic transmission is upshifted, whereby the number of rotations of the drive wheels is further increased to exert an influence to the durability of a differential or the like.
A control system is known from Japanese Patent Publication No. 64-7259, which is designed, so that when the rotational speed of the rotatable member becomes equal to or larger than a predetermined value, it is determined that the drive wheels are in a slipping state, thereby prohibiting upshifting. In the above known control system, however, even if the cancellation of the prohibition of the upshifting is not precisely carried out, there is a possibility that the prohibition of the upshifting is continued, or the prohibition of the upshifting is canceled before the elimination of the slipping state.
To solve such a problem, a control system has been proposed for an automatic transmission for a vehicle such as in Japanese Patent Application No. 8-327015, and which is designed so that it can detect with a good accuracy, that the slipping state of the drive wheels is eliminated to restore the gripping between the drive wheels and the road surface, whereby the prohibition of the shifting can be precisely canceled.
When a vehicle provided with such a control system for the automatic transmission is started on ice which has a small friction coefficient, if the slipping is generated immediately after the starting with a first gear shift stage established, an electronic control unit which has detected the slipping can provide time to carry out a control for prohibiting the upshifting from a second gear shift stage to a third gear shift stage. However, if slipping is generated during the upshifting from the first gear shift stage to the second gear shift stage, the upshifting from the second gear shift stage to the third gear shift stage is immediately started, subsequent to the completion of the upshifting from the first gear shift stage to the second gear shift stage. For this reason, there is a possibility that if the electronic control unit which has detected the slipping tries to carry out the control for prohibiting the upshifting from the second gear shift stage to the third gear shift stage, the control is not in time and hence, the undesirable upshifting from the second gear shift stage to the third gear shift stage is carried out.